In a cellular communication system, one base station performs radio communication with a plurality of communication terminals simultaneously, and therefore, as demand has increased in recent years, so has the need for higher transmission efficiency.
One technology that has been proposed for increasing the transmission efficiency of a downlink from a base station to a communication terminal is HDR (High Data Rate). HDR is a communication method whereby a base station performs scheduling for allocating communication resources to communication terminals by time division, and also sets a transmission rate for each communication terminal according to the downlink channel quality.
The operations by which a base station and communication terminals perform radio communication with HDR are described below. First, the base station transmits a pilot signal to each communication terminal. Each communication terminal measures the downlink channel quality using a CIR (desired signal to interference ratio) based on the pilot signal, etc., and finds a transmission rate at which communication is possible. Then, based on the transmission rate at which communication is possible, each communication terminal selects a communication mode, which is a combination of packet length, coding method, and modulation method, and transmits a data rate control (hereinafter referred to as “DRC”) signal indicating the communication mode to the base station.
The type of modulation method that can be used in each system is predetermined as BPSK, QPSK, 16QAM, 64QAM, and so forth. Also, the type of coding that can be used in each system is predetermined as 1/2 turbo code, 1/3 turbo code, 3/4 turbo code, and so forth. Further, a plurality of transmission rates that can be used in each system are predetermined according to a combination of packet length, modulation method, and coding method. Each communication terminal selects a combination whereby communication can be performed most efficiently with the current downlink channel quality, and transmits a DRC signal indicating the selected communication mode to the base station. Generally, DRC signals are represented by numbers from 1 to N, with a higher number indicating a proportionally better downlink channel quality.
Based on the DRC signal transmitted from each communication terminal, the base station sets a transmission rate for each communication terminal, and sends a signal to each communication terminal via a control channel indicating communication resource allocation to each communication terminal. The base station then transmits data only to the relevant communication terminal in its allocated time. For example, if time t1 has been allocated to communication terminal A, in time t1 the base station transmits data only to communication terminal A, and does not transmit data to a communication terminal other than communication terminal A.
In this way, data transmission efficiency has conventionally been increased for the overall system by setting a transmission rate for each communication terminal according to channel quality by means of HDR, and performing communication resource allocation preferentially to a communication terminal with a high transmission rate at which communication is possible.
However, as downlink channel quality measurement in a communication terminal is performed based on the pilot-part signal within a received signal, if the length of the pilot-part signal is short compared with the length of the data-part signal, a difference may arise between the measured channel quality and the current channel quality due to the effect of fading, etc., while the data-part signal is being received. As communication mode selection is performed based on the measured channel quality, when such a difference arises there is a problem in that the communication mode in which communication can be performed most efficiently with the current channel quality will not be selected, and downlink throughput will fall.
Also, if error occurs in the channel quality measurement circuit, a difference will arise between the measured channel quality and the actual channel quality, and the same kind of problem as described above will arise.